Signal delay device



= Feb. 6, 1951 MAN KIN SIGNAL DELAY DEVICE Filed March 7, 1947 atenteci 6,

SIGNAL. DELAY DEVICE Arthur H. Mankin, Philadelphia, Pa., assigner to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania,

Application March 7, 1947, Serial No.73 2,999

3 Claims. (Cl. 8e-44) This invention relates to an improved signal comparator for use in comparing the waveforms cf Successive portions of an electrical wave signal or for comparing the waveform of an electrical signal occurring at a given time with that of another signal occurring at a different time. It relates also to an improved signal delay device suitable for eiectively delaying electrical signals for ,long intervals of time.

According to one known method of comparing the waveforms of signals occurring in timespaced relation, the earlier signal is delayed in time by an amount sufficient-to make it contemporaneous with the later signal. The delayed earlier signal is then subtracted from the later' occurring signal, or vice versa, and any resultant signal is noted as indicative of a difference in waveform between the earlier and later signals.

The principal limitation upon this method is imposed by the magnitude of the time-spacing between the signals toV be compared. Where only small time-spacings are involved, the required delayvis rather readily obtainable through the use of conventional passive electrical delay networks employing either lumped or distributed parameters. However, when long delays (of theV order of a millisecond or more) are required, these means are no longer suitable because of their inherent dissipation.

One method by which it has been attempted to secure greater delays involves the use of a column of a medium such as mercury suitable forthe propagation of compression waves at supersonic frequencies. At one end of the column there are created in the medium (e. g. bymeans oi' a crystal transducer) disturbances at a relatively high frequency, whose amplitude is caused to vary in accordance with variations in the amplitude of thesignal to be delayed. Thus there are propagated in the medium compression waves comprising 'successive condensations and rarefactions at the exciting frequency. These waves are transmitted through the column to the other end thereof, where they are again converted (e. g. by means of a second crystal transducer) into electrical signals corresponding in form to the signals originally used to setup the disturbances at the opposite end of the column but delayed in time with reference thereto. These signals will comprise a high frequency carrier modulated by a signal ofwaveform corresponding to thatv of the delayed input signal. The modulation may be detected in any well known manner and is then available for comparison with any contemporaneously occurring` signal.

, This arrangement is subject to the disadvanl tage, among others, that the crystal transducer: employed must be maintained in extremely accurate alignment4 (i. e. the departure of theii mechanical axes from parallelism preferably should not exceed one-half wavelength, at the' frequency of the supersonicv waves ,propagatedi1; the medium, over a length equivalent to the sepa, ration between the crystals) in order that sat-` isfactory pickup of the delayed signal may b obtained at the end of the column remote from the point of propagation. Such alignment is eigceedingly dicult to maintain in practice, and it is required, therefore, that the arrangement be continually adjusted in order to insure satisfactory operation.

Accordingly the principal objects of the invention are as follows:

(l) to provide an improved signal waveform comparator `requiring a minimum of adjustment in operation;

(2) to provide' improved means for comparing the waveforms of electrical signals occurring in relatively large time-spaced relationship; (3) to provide improved means for effectively delaying electrical signals for relatively long in--l tervals of time without undue attenuation and without apprciable alteration in waveform;

' (4) to provide an improved signal delay device in which the amount of delay is readily and con-jfveniently adjustable throughout either small or large ranges; I

5) to provide an improved signal delay devicei` comprising a column of a medium in which suf" personic Waves are propagated in response t0: signals to be delayed and signals are derived in"l response to ldisturbances in the medium at'a point remote` from that of propagation, said de-` vice requiring a minimum of adjustment of the means used to propagate said waves and themeans used to derive said signals; and

(6) to provide an improved signal delay device comprising -a column of a light transmissive medium in which supersonic waves are propagated-l in response to signals to be delayed and in which optical means are employed to derive a signal in response to disturbances in the medium at ai point remote from that of propagation.

Other objects and features of the invention will become apparent from a consideration of the following description and the appended pression waves are propagated in a column of a medium in the same manner as in the system just referred to. However, instead of mercury, water or some equivalent light-transmissive medium is employed. A beam of light is produced and is caused to traverse a region in the medium in the vicinity of the end of the column remote from that at which the waves are propagated. Upon emerging from the medium, the light beam is focused so that, in the absence of disturbances in the region of the medium through which it has passed, the emergent rays will he focused to impinge upon a stop. Disturbances propagated in the medium are, however, adapted to diffract the rays of the beam of light so as to cause a portion of the light to pass on either side of the stop. From the diffracted rays are selected, by means of a second stop provided with an aperture, only those rays which passed through a relatively limited region in the medium. The rays thus selected are permitted to impinge upon a photoelectric cell, whereby there are generated signals which vary in response to variations in the disturbances in the medium in the region through which the' impingent rays have passed. The signals` thus generated will be delayed versions of those'in response to which disturbances in the medium were propagated. It will be apparent: that the invention can be practiced as briefly set forth above, using either light in the visible portion of the spectrum or, as well, light in the infra-red or ultra-violet regions, for example, which is invisible. Hence the term light," used in the annexed claims, is to be construed as meaning either visible or invisible light.

A more complete understanding of the invention and of the mode of operation of a representative embodiment will be obtained by reading the following detailed description with reference to the accompanying drawings, in which:

Figure 1 illustrates a representative embodiment of the invention, and

Figure 2 illustrates the application of the method and means of the invention to a radar target motion indicating system.

Referring now to Figure. l, input signals are supplied through connections lA and 2 to modulate an oscillator 3. -These signals may comprise either a continuous signal, successive time-spaced portions of which are to be compared as to their waveform, or they may comprise discrete signals occurring in time-sp-aced relationship, whose waveforms are to be compared. Moldulated oscillator 3 may comprise circuits of conventional form including a piezo-electric crystal 8 connected therein through connections 4 and 5 to its associated electrodesl 6 and 1 respectively. 'I he amplitudesof the oscillations generated by it are modulated in response to the input signals supplied to it through connection 2'. The frequency of the oscillations generated by oscillator 3 should be sufficiently high to accomm date the sidebands which will be produced in response to modulation by the input signal. Thus, where the signal comparator is to be used in a radar target motion indicator, this frequency may be of the order of ten megacycles, and is determined by appropriate selection of the natural frequency of crystal 8 and of the magnitudes of the other components comprising the oscillator circuit, as is Well known in the art.

Crystal 8 and its associated electrodes B and 1 are disposed at one end of a tubular container 9 Which is conveniently made entirely of glass or' some equivalent light transmissive material, a1-v though it is to be understood that only the upper portion of the container, through which light is to be transmitted,.need be transparent. Hence it would be feasible to employ an opaque container having transparent windows appropriately disposed to permit the passage of a beam of light. The interior I of the container 9 is lled with a light transmissive medium, such as water, suitable for the propagation of supersonic compressional waves in response to the vibrations of crystal 8. These waves, which will vary in intensity in accordance with variations in the amplitude of the input signals, are propagated through the medium longitudinally of the tubular container 9 in the-direction indicated by the arrow lila. The end of the container opposite that at which the crystal is disposed is preferably terminated in an absorber Il filled with a material such as cotton adapted to absorb the propagated waves and substantially prevent reilection thereof.

At any time after the first wave of a train propagated in response to vibrations of crystal 8 has traversed the length of the tubular ccntainer 9, there will be present along the entire' length of the column of the propagating medium HJ within the tube, at any instant of time, a distribution of compressional disturbances (i. e; ccndensations and rarefactions) corresponding in their intensities to the amplitudes, at various instants, of the modulated carrier wave input to. crystal 8 during the preceding interval of dura-1 tion equal to the time required for a wave to. travel the length of the tube. Thus, if the rnedium is water having a velocity of propagation of. 1500 meters per second, a column 1.5 meters in length would comprise a train of condensations and rarefactions corresponding to variations in the input signal during the preceding millisecond. Those in the immediate vicinity of the crystal will correspond in intensity to the amplitude of the signal then being applied to the crystal,` while those in the vicinityv of the en d of .the'

column remote from the crystal will correspond to the amplitude of the signal applied one millisecond before. A A y ,l-

If a ray of light is transmitted through the, column of the propagating medium in a direction, transverse to the direction of propagation, it is,l found that it is diffracted to a degree dependent upon the intensity of condensation or rarefaction in the plane through which it passes. 'This phenomenon is not in itself new but hasvpreviously been investigated and usefully appl'ed, for example in a television receiving system as described in 27 proceedings of the Institute-of Radio Eni. gineers 483, et seq. The present invention utilizes. the phenomenon to derive a signal dependent' upon the degree of condensation or rarefactionI in a particular plane in tle medium transverse to the direction of propagation and having a predetermined displacement from the point at which waves are propagated. The-signal thus derived:- is therefore dependent upon-the ampltde of an'I input signal at a timefprior by a predeterminedamount to the time at which the signal is derived. A l ,l

As illustrated in Figure 1, light from a suite` able high intensity source I 3, such as an "arc, discharge, is collimated by means of lens I4 to produce a parallel-raybeam oflight'ofconvenlent width, which is transmitted through the. walls of container 9, and through the mediumv 'to therein contained, `in. a direction transverse"to,l

the longitudinal axis. of the container. The light source I3 and lens 4I4 are-sol arranged that; some of the rays of the light beam shall liei'in the plane I2,- transverse tothe axis of tube-f 9, in, which occur the compressional disturbances response to which 'an output signal is tojbe generated. Thus it maybe arranged, for v ex-- ampia that the central rays of the beam traverse the medium in this plane. As hereinbefore mentioned this plane is displaced from the plane of propagation by an amount corresponding to the desired delay of the output signal with reference to the input signal in responseA to which the compression waves are propagated. 'Ijhe displacement determines the amount by which the output signal is delayed with reference to the input signal, and is limited only by.: the dissipation in the medium. For ten megacycle waves in water this dissipation amounts to about six decibels per meter, so that no particular problem is presented in obtaining delays. of a niillisecond or more.

Eer the purposes of the invention, only the rays of the beam passing through the medium in' the immediate vicinity (e. g. within a. fraction of a wavelength at the carrier frequency) ofplane I2 are of interest. .The reason for this will later become apparent. However it would not bejeasible to limit the beam to such a narrow Width, and hence abeam of any desired convenient width may be used, subsequent provision being made to restrict the information contained in the output signal to correspond to dise turbances only in the immediate vicinity of the particular plane selected.

'To increase thel amount of light which is subjectto diffraction in response to compressional disturbances in the vicinity of plane I2, the width of tube 9v may be increased in the direction transverse. to the axis of the light beam, and lens I4 may be made of suitable form to distribute the light from source I3 in parallel rays in a beam of comparable width in this di. rection.

Upon emerging 9'the light. comprising the beam passes through another lens I5. If the emergent rays are parallel. as will be the case if no disturbances exist. inthe region in the medium traversed by the beam. lens I5 is adapted to converge all of the light in the beam upon a. stop I8. However, if disturbances exist in this region, some of the emergentrays will be diracted and will not bev interceptd by stop I6. 'I'hese diiracted rays are 4focused by means of lens I'I to form, in the plane of stop I8, an image of the region of the wall of tube 3 :from which the beam of light emerges. This image will comprise variations in light intensity throughout its length corre.- sponding to variations in thev amplitude of the disturbances. within thev region in the medium in tube 9 traversedby the light baarn. More particularly the intensity in the center ofthe image will be a function of the disturbances. ex-

istingin the region of plane I2 in the. medium. Accordingly there is provided an aperture or slit I9 in stopV IB which is adapted to transmitv only the light which has passed through the medium in the immediate vicinity of plane I2. This light is permitted to impinge upon the photoelectric cathode of photoelectric cell. 23 to. produce electrical signals which vary in intensity in response to variations in the intensity of disturbances. in the vicinity of plane I 2. These,

signals will therefore. correspond todelayed ver- '15 from the opposite wall of tube.

plied in opposite phase,l the delayed output from amplifier 2| is subtracted from the undelayed output from connection 24, and in the event thatthese two signalsare not substantially identical in waveform, the output from the subtracr. tor will serve to indicate this fact.

Reference has already been made to the fact1 that only those rays of the light beam are. of. interest which traverse the medium in a limited'fv region which extends but a fraction of a Wavelength on either side oi plane I2. Accordingly-vthe width of aperture I9 should be made narrowg enough to transmit to photocell 20 only -thesel rays. If rays outsidethis limited region are per-f mitted to reach the photocell, the output signal will not be related, as desired, to the amplitude` of the input signal at a particular instantofA time, but rather will be relatedto the average 0f the amplitude of the input signal during an.v

interval whose duration is related to the Width'.

of the aperture. Although the exact width .of

the aperture, in any given instance, can best-bef. determined by experiment, the best results appear to be obtainable when it is of suilicientl width to pass the diffracted rays from a region inthe medium' which is. l.ss. than one-half wavefv length in width at the carrier frequency.

It will be apparent. that lateral displacement" of aperture I9 in stop I8 with reference to the. provides a convenient.

optical axis of the system means of effecting small adjustments in the delay to which the input signal to the supersonic delay'-v it will be evident that.'`

cell is subjected. Likewise larger variations in delay may be eiected by vdis,-

placement laterally of the axis ofthe optical sysj tem as a whole;

In Figure 2, which illustrates diagrammatically A a complete radar target motion indicator systern.'` embodying a signal comparator in accordance` with the'invention, modulator 30 is adapted' to.'

generate time-spaced pulse signals which are supplied to magnetron oscillator 3l to control the: generation of similar time-spaced pulses of ra' dio frequency energy.v These pulses are supplied' through Ts-R box 32 to an antenna 33 for trans.-v mission. Reflections of the transmitted pulses 'y from target objects are also intercepted by antennav 33 and are supplied through T-R box 32 to the` receiver circuits of the radar ner 34.

T-R box 32 operation of the latter and possible destruction of certain of their delicate components. `In one of its usual forms thisl device may -comprise 'af' resonant discharge path enclosed in an evacuat-l ved envelope, the discharge path being connectedf eiectively in series-with the energy transmission' channel from the transmitter to the antenna and-`l in shunt with thatfrom the antenna to -the Trer-- ceiver, preferably at a point where the two join. The discharge path is arranged so as to conduct system, compris? ing converter and intermediate frequency ampli'- lis a conventional device,` wel'lf" known in the radar art, for the purpose of alternately permitting the passage of energy from osci1-lator3| to antenna 33, and of received energy 1 from antenna 33 to converter and I.-F. amplifier;v 34, while minimizing the amount of energy from the magnetron oscillator reaching the receiver" circuits, so as to prevent interference'with the "egal-asiat in response to large bursts of energy to the trans mitter but notin response to relatively smaller amounts of energy proceeding from the antennatoward the receiver.

r The converter section of converter and I.F. amplifier 34' is adapted to convert the received modulated carrier wave signal to an intermediate carrier frequency suitable for application to the crystal transducer of a supersonic delay tube of the sort just described with reference to Figure "1. This tube together with its associated optical means and photoelectric pickup cell, which may be arranged in the same manner as described With reference to Figure 1, comprises` delay means 36 of the arrangement presently being described. The delay produced by these means, as determined by the length of the super'` sonic delay tube, is made equal to the desired in terval between pulsesV of radio frequency energy to be transmitted by the radar system. The output from the photoelectric cell included in delay means 36 may be supplied to modulator 3B to control the spacing of the control pulses generated thereby. Thus, for examplemmodulator. 3B Amay be a device, such as a non-oscillating' multivibrator, which is capable of generating modulating pulses only in response to triggering pulses supplied to itfrom the output of delay means 36. Such triggering pulses may comprise the portion of each transmitted pulse which en ters the receiver circuits, as delayed in delay means 36. Then modulating pulses will be produced at intervals substantially equal to the delay provided by delay means 36. The output from delay means 36 is also supplied through an amplifier 31 to a subtractor circuit 38 as in the arrangement according to Figure 1. Also the output of converter and I.F. amplier 34- is supplied to detector 39 from which is derived an undelayed signal corresponding to the envelope of the received radar reiiection from a given transmitted pulse. In subtractor 38 this u ndelayed signal is subtracted from the delayed signal from the output of amplifier 31, which.

corresponds to the next preceding transmitted pulse. The resultant signal is supplied toa suitable indicator lill which may be the same as that used in any conventional radar system. Whenever the return signals from-successively transmitted radar pulses diier, owing for example to the motion of a target Within rangev of the radar, indicatorl is therefore adapted toprovide an indication of such motion.

.It will, of course, be understoodthat the invention is susceptible of embodiment in forms other than the one here shown and described and that it is not limited in its application to any particular class of systems, such. as radar target motion indicator systems herein referred tov to illustrate its application. Rather it is applicable Wherever signals occurring in timespaced relation are to be delayed for relatively long time intervals, and the scope of the invention is subject only to those limitations irnposedby the following claims.

I claim:

,p-l. In a delay device for electrical wave signals:

aimedium adapted to transmit mechanical com;

pressional waves and comprising at' least a re- Waves whose amplitude varies in rsp'se to saidl input signals; means for directing light through said region in a direction substantiallyv trans'-l verse to the direction of propagation of Waves therein; means including a light stop constructed'v and arranged to intercept at least a portion of the light emerging from said region, the magnitude of the portion so intercepted varying in accord-i" ance with the degree of diffraction of said lightV in response to compressional disturbances in said region; means for focussing light which is not intercepted by said stop to form an image of said region, a second light stop disposed substantially in the plane of said image, said second stop being provided with an aperture of dimensions su1i "1-;-t cient only tc permit the passage of light which, in traversing said region, comprised a beampf Width substantially less than a Wavelength-'of said propagated Waves in the direction of their! propagation, and means for deriving an output;` signal which varies in accordance with the vari-' ations in light passing said second stop.

2. In a delay device for electrical wave signals;4 a medium adapted'to transmit mechanical com# pressional waves and comprising at least aregioir which is light-transmissive and whose light dif-ji fractive properties are dependent upon the inten-,j sities of compressional disturbances therein;v means responsive to input signals which are to be delayed for propagating in said medium, at a point d splaced from said region, compressional waves Whose amplitude varies in response to said; input signals; means for directing light through: said region in a direction substantially trans-' verse to the direction of propagation of -waves therein; means including a light stop interposed in the path of light emerging from said regiorp; said last-named means being constructed and ar; ranged so as normally' to intercept substantially.. all of said emergent light in the absence of com-j,l pressional disturbances in said region, but being' adapted to permit the passage of light diiracted in response to compressional disturbances in said region; means for focussing said diiracted light' tov form an image of said region in a plane dis-1 placed from said stop; a second light stop dis' posed substantially in the plane of said image.; said second stop being provided with a slit of width suiiicient to permit it to pass only light which has traversed a predetermined portion of said region Whose dimension in the direction' of propagation of said waves is substantiallyless than a wauelength of said propagated waves; and

means responsive to light passing said'nsecond4 s top for deriving delayed versions -of said inputj Vil signals.

' 3. In a delay device for electrical Wave signals,

a medium adapted to transmit mechanical com-.

pressional Waves and comprising at least a region which is light-transmissive and whose light' difJ-f Wavesl whose amplitude varies in response ,to said.,

input signals; means for directing light through said region in a direction substantially transverse to the direction of propagation of waves therein... and in a beam Whose dimension in the direction,

of propagation of said Waves issubstantially;l greater than a wavelength of said propagated, Waves; a light stop interposed in thev path4 of;l

said beam at a point subsequent to the emergenceI .of said beam from said region, the dimension of l said stop being substantially less than the dimension of said beam in the direction of `propagation of` said waves; means for focussing said light emergent from said region to cause a substantial portion thereof to be interceptedby said stop, the magnitude of said portion being subject to variation in accordance withthe degree of diffraction of said light in responseto compressional disturbances in said region; means for focussing llight which is not intercepted byjs'aid stop to form an image of said region, meandisposed substantially in the plane of said ima which, in traversing said rwgvvgoncomprised a beam of width substantially'less than a wavelength of said propagated waves in the direction.

of their propagation, and meangfor deriving an output signal which varies inaccordance with the variations in said selected light.

ARTHUR l0 REFERENCES CITED UNITED STATES PATENTS Number y Name Date 1,678,653 Schroter July 31, 1928 1,694,661 Meissner Dec. 11, 1928 2,155,659 Jeiiree Apr. 25, 1939 2,155,661 Jeiree Apr. 25, 1939 2,287,587 Willard June 23, 1942 2,418.964 Arenberg Apr. 15, 1947 2,422,135 Sanders, Jr. June 10, 1947 

